A polytetrafluoroethylene (hereinafter referred to as PTFE) is excellent in heat resistance, chemical resistance, weather resistance, a gas barrier property, etc. and used in various field, such as semiconductor industry and automobile industry. When commercially available PTFE is melted, its molten viscosity is so high that conventional melt molding cannot be carried out. Therefore, in order to mold PTFE, molding methods such as paste extrusion molding using a process aid, compression molding and ram extrusion have been employed. As a melt-moldable tetrafluoroethylene copolymer, a copolymer (hereinafter referred to as PFA) of tetrafluoroethylene (hereinafter referred to as TFE) with a perfluoro(alkyl vinyl ether) (hereinafter referred to as PAVE) has been known. In a case where PAVE is a perfluoro(propyl vinyl ether) (hereinafter referred to as PPVE), based on the total mass of repeating units based on TFE and repeating units based on PPVE in PFA, the content of repeating units based on TFE is from 94 to 97 mass %, and the content of repeating units based on PPVE is from about 3 to 6 mass %. Since production cost of PAVE is high, production cost of PFA is also high. Further, the melting point of PFA produced by copolymerizing PPVE is usually at most 310° C., and the heat resistance is low as compared to PTFE.
Recently, development of melt-moldable PTFE has been reported (for example, Patent Documents 1 and 2). This PTFE is considered to be a copolymer of TFE with a trace amount of another fluoromonomer. This PTFE has a higher melting point than PFA and thus is expected to be useful also under a higher temperature condition. Further, since the content of repeating units based on the fluoromonomer is less than the content of repeating units based on PAVE in PFA, it is considered that the production cost of the PTFE is lower than the production cost of PFA. However, in Patent Documents 1 and 2, the detail of the method for producing a melt-moldable PTFE is not described.
Usually, as the method for producing PTFE, a radical polymerization method is employed, and particularly a suspension polymerization method which uses no solvent or an emulsion polymerization method which uses a fluoroemulsifier is employed. On the other hand, as the method for producing PFA, a suspension polymerization method which uses a fluorine type solvent or an emulsion polymerization which uses a fluoroemulsifier is employed.
In the method for producing the above melt-moldable PTFE, it is considered that like the method for producing PFA, a suspension polymerization method which uses a fluorine type solvent or an emulsion polymerization method which uses a fluoroemulsifier may be used. However, in recent years, from the viewpoint of environmental protection, use of fluorine type solvents or fluoroemulsifiers tends to be restricted, and in the production of a melt-moldable PTFE, it is preferred to employ a suspension polymerization method using no fluorine type solvent.
However, it has been found that if a melt-moldable PTFE is produced by the suspension polymerization method using no fluorine type solvent, the molecular weight becomes too high, and a melt-moldable property tends to be lost. Therefore, it is conceivable that at the time of producing the melt-moldable PTFE, a chain transfer agent is added in order to control the molecular weight of the melt-moldable PTFE. However, it has been found that if as the chain transfer agent, an alcohol such as methanol as used for PFA or hexane is used, the polymerization rate becomes low, and thereby mechanical properties such as tensile strength of the melt-moldable PTFE deteriorate, and coloration occurs.    Patent Document 1: JP-A-2003-523436    Patent Document 2: JP-A-2003-520863